The present invention relates to the lighting arts. It especially relates to high intensity light emitting diode packages, components, apparatuses, and so forth, and will be described with particular reference thereto. However, the invention will also find application in conjunction with other solid state light emitters such as vertical cavity surface emitting lasers.
High brightness light emitting packages typically employ a plurality of light emitting diode chips, surface emitting laser chips, organic light emitter chips, or the like. To mechanically support the chips and to electrically interconnect the chips, in some light emitting packages the light emitting chips are disposed on a printed circuit board. The printed circuit board can also support and electrically incorporate discrete electronic components, application-specific integrated circuits (ASICs), programmable microprocessors, or the like, for providing input power conditioning, light output control, electrostatic discharge protection, or other functions.
Disadvantageously, the printed circuit board can contribute to optical losses by partially absorbing light that impinges upon the printed circuit board. Printed circuit boards typically include a topmost epoxy solder mask layer having lithographically defined openings through which the light emitting chips or other electronic components electrically contact bonding pads of the printed circuitry. In conventional printed circuit boards for electronic applications, the solder mask layer is not optimized for its optical properties, and is thus not very reflective.
For high brightness light emitting packages, printed circuit boards having a commercially available white solder mask are sometimes used. These white solder masks contain white talc or another white material that reflects visible light. White solder masks provide a substantial improvement in reflectance of visible light over conventional blue or green solder masks.
White solder masks have certain disadvantages for some high brightness light emitting packages. While the white solder mask appears to be highly reflective, it has been found that the reflectance of such boards is only about 80% or less in the visible spectral region. If 50% of the visible light produced by the light emitting package impinges on the printed circuit board, this reflectance corresponds to optical losses of around 10% or higher due to absorption in the printed circuit board.
Moreover, the reflectance of the white solder masks decreases in the blue, violet, and ultraviolet spectral regions. It has been found that the white solder board reflectance decreases below 60% for wavelengths less than about 410 nm. In certain light emitting packages, wide-bandgap light emitting chips emitting blue, violet, or ultraviolet light emission are coupled with a phosphor that converts the light emission into white or another selected visible light. In such packages, a substantial amount of blue, violet, or ultraviolet light typically reflects from the phosphor toward the printed circuit board. The relatively low reflectance of white solder masks for blue, violet, or ultraviolet wavelength light degrades the light output efficiency of these packages.
White solder masks have been constructed to have a reflectance of about 85% between about 400 nanometers and about 740 nanometers. These white solder masks have been formed of a film of photostabilized electrically insulating binder material, such as a photosensitive epoxy, a photosensitive polyimide, or so forth, in which particles of a reflective filler material are dispersed. The particles used for the reflective filler material include a titanium dioxide, TiO2, alumina, Al2O3, and a high-refractive index glass.
Another known material used as a solder mask is formed of a multiple-layer specular reflector sheet. Openings are cut into the sheet to define the vias, and the multiple-layer specular reflector sheet is glued or otherwise secured to the electrically insulating board. This reflector sheet includes a stack of reflective dielectric layers including alternating layers of dielectric material having different refractive indices, different birefringence characteristics, or other dissimilar optical characteristics. The thicknesses and optical characteristics of the layers has been selected to provide a high stack reflectance in the wavelength range between about 400 nanometers and about 470 nanometers, and preferably in the wavelength range between about 400 nanometers and about 740 nanometers. The multiple-layer specular reflector sheet that has been described is a commercially available reflector sheet, such as a VM2000 reflector film or a Vikuiti™ enhanced specular reflector film (available from 3M, St. Paul, Minn.). Solder masks formed of VM2000 or Vikuiti™ enhanced specular reflector films have been measured to have reflectances greater than 85% between 400 nanometers and 740 nanometers.
An approach for achieving high reflectances in the blue, violet, or ultraviolet would be to use a metallic reflector. However, incorporating a metallic reflector, which is generally electrically conductive, into a solder mask is problematic since the solder mask is in contact with or in proximity to the printed circuitry and electronic component leads.